Group III nitride semiconductors, such as notably gallium nitride (GaN), are wide gap semiconductors. For example, gallium nitride (GaN) and aluminum nitride (AlN) have energy gaps of as large as 3.4 eV and 6.2 eV, respectively, at room temperature. The Group III nitride semiconductors also have large breakdown electric fields, and higher electron saturation velocities than those of compound semiconductors such as gallium arsenide (GaAs) and the like, silicon (Si), and the like. Due to these properties, field effect transistors (FETs) made of a GaN-based compound semiconductor material, which are used as high frequency electronic devices or high power electronic devices, have been vigorously studied and developed.
Nitride semiconductor materials, such as GaN and the like, can be used along with AlN or indium nitride (InN) to produce various mixed crystals, and therefore, can provide a heterojunction as with conventional arsenide-based semiconductor materials, such as GaAs and the like. The heterojunction provided by the nitride semiconductor (e.g., an AlGaN/GaN heterostructure) has a property that a high concentration of carriers which are generated by spontaneous polarization and piezoelectric polarization occur at the interface without doping with an impurity. Therefore, when an FET is made of the nitride semiconductor, the FET is likely to be of the depletion mode type (normally on type), but not of the enhancement mode type (normally off type). However, because most of the devices currently used in the field of power electronics are of the normally off type, there is a strong demand for normally off GaN-based nitride semiconductor devices as well.
Normally off transistors are caused to be normally off by, for example, the following reported methods: a so-called recess structure is formed in which the AlGaN layer of the AlGaN/GaN structure is thinned only in a portion below the gate electrode to reduce the concentration of two-dimensional electron gas (2DEG), thereby shifting the threshold voltage to a positive value; and the GaN layer having a plane orientation {11-20} is grown on a main surface of a sapphire substrate which has a plane orientation {10-12}, thereby reducing or preventing the generation of a polarized field in a direction perpendicular to the main surface of the sapphire substrate. Here, a minus sign “−” added to a Miller index in a plane orientation indicates the reciprocal of the index following the minus sign for the sake of convenience.
As a promising structure which can provide a normally off FET, a junction field effect transistor (JFET) has been proposed in which a p-type AlGaN layer is formed in a portion where the gate electrode is formed. In the JFET structure, the p-type AlGaN layer is connected to a barrier layer made of AlGaN so that the potential energy of the AlGaN layer is raised. As a result, the concentration of two-dimensional electron gas formed immediately below the gate electrode formation portion in which the p-type AlGaN layer is formed, can be reduced, whereby the JFET is allowed to operate in a normally off mode. Moreover, because a pn junction, which has a higher built-in potential than that of a Schottky junction between a metal and a semiconductor, is used in the gate electrode formation portion, the rising voltage of the gate can be increased. Therefore, even when a positive gate voltage is applied, the gate leakage current can be limited to a small level.
Note that, here, AlGaN denotes AlxGa1-xN where x is 0<x<1, InGaN denotes InyGa1-yN where y is 0<y<1, and InAlGaN denotes InyAlxGa1-x-yN where x and y are 0<x<1, 0<y<1, and 0<x+y<1. This expression is also used in the descriptions below.